1. Field of the Invention
The present invention relates to a controller for a hybrid front-and-rear-drive automotive vehicle wherein an engine and an electric motor are provided as two drive power sources for operating one and the other of front and rear wheel drive systems, respectively.
2. Discussion of the Related Art
There is known an automotive vehicle of a type wherein an internal combustion engine is used to drive one of a front wheel drive system and a rear wheel drive system while an electric motor is used to drive the other of the front and rear wheel drive systems. In this type of automotive vehicle, all of the four wheels can be driven by concurrent operations of the engine and the electric motor. For instance, the electric motor is activated to drive the wheels of one of the front and rear wheel drive systems while the wheels of the other drive system is operated by the engine. In this respect, the vehicle of the type in question may be called a hybrid 4-wheel-drive vehicle. For improved overall driving performance of this hybrid 4-wheel-drive vehicle while maintaining high degrees of fuel economy and other running characteristics of the vehicle, the electric motor is activated to assist the engine, namely, to provide an engine-assisting motor drive torque only where the vehicle is placed in a predetermined running condition requiring acceleration of the vehicle.
In the field of such a hybrid front-and-rear-drive vehicle, it has been proposed to provide an electric generator operable by the engine for generating an electric energy used for operating the electric motor, in an attempt to eliminate a high-voltage battery for operating the electric motor, and reducing the size of the electric motor. An example of such a hybrid front-and-rear-drive vehicle is disclosed in JP-A-8-126117.
In the hybrid front-and-rear-drive vehicle wherein the electric motor is operated by an electric energy supplied thereto directly from the electric generator driven by the engine, as described above, however, an operation of the electric motor is more or less delayed since it takes a time for the electric energy to rise to a level sufficient for starting the electric motor. Accordingly, an increase of the engine-assisting torque generated by the electric motor is delayed. Where the vehicle is started on an ascending road surface having a relatively high gradient, by concurrent operations of the engine and the electric motor, the delayed increase of the engine-assisting motor torque will unfavorable lead to initial slipping of the wheels driven by the engine on the ascending road surface.